Add get_band_fluxes to source models

[OliviaLynn]

We should add a method in source models that:

• takes in a Passband or PassbandGroup (and time t)
• evaluates the model based on the Passband/PassbandGroup's waves array to get spectral fluxes
• call the Passband/PassbandGroup's flux_to_bandflux/fluxes_to_bandfluxes on these spectral fluxes
• returns the band fluxes

Based on Mi's description:

I left a comment on Olivia's passband PR. After looking at the code, I would like to keep advocating for the following interface:
passband = Passband(delta_wave: float | None = 5, trim_percentile: float | None = 0.1) band_flux = source.get_band_fluxes(passband, t)

This will avoid confusion from users having to call spectral_flux = source.evaluate(passband.waves, t) and having to know that the evaluate should be on the passband.waves grid. And not having to check if user has actually call evaluate with the correct grid.